"It is very clear that, as an Irish nationalist, Casement was motivated by a deep sense of the injustice he witnessed in the suffering which affected many in Irish society and an equally strong belief in the right of the people of Ireland to decide their own futures," he said.

"This belief was shared by the other 15 executed leaders (of the Rising) and indeed all of the men and women who took part in the Rising and the subsequent journey to Irish statehood.

"Now, 100 years on, we are committed to live up to the ideals and aspirations of Casement and these men and women, in particular their aspiration for an Ireland that 'declares its resolve to pursue the happiness and prosperity of the whole nation'."

Before helping organise the Easter 1916 rebellion against British rule, Casement was a distinguished British public servant, knighted for his ground-breaking work exposing cruelty against native labour in imperial colonies.

Deeply affected by the plight of his fellow Irish - he was born in Dublin and raised in Antrim - he later helped organise the uprising, which failed militarily but ultimately led to the partition of Ireland, and independence for the Irish Republic.

Arrested at Banna Strand in Co Kerry on April 24 1916 amid a botched gun-smuggling operation from Germany, he was tried on charges of high treason and hanged at Pentonville Prison.

Nearly 50 years later his remains were repatriated to Ireland for a State funeral at Glasnevin cemetery in Dublin.

Speaking at the graveside, Mr Flanagan said Casement's life work left a lasting legacy both in Ireland and overseas.

"Looking back at Casement's life with all the benefit of historical reflection, perhaps what stands out most is the generosity of spirit and selflessness which led him to take a central role in the Irish independence movement as well as to spend much of his life in the African and South American continents campaigning against the terrible abuse of the rights of people happening there at the time," he said.

"For Casement, these endeavours were not two separate parts of his life."

Ireland's Defence Forces and overseas humanitarian aid agency Irish Aid also marked the centenary with an open day at the military aerodrome in Baldonnel, west Dublin, renamed in his honour.